Abiotic environmental stresses, such as drought, salinity, heat, and cold, are major limiting factors of plant growth and crop yield. Crop yield is defined herein as the number of bushels of relevant agricultural product (such as grain, forage, or seed) harvested per acre. Crop losses and crop yield losses of major crops such as soybean, rice, maize (corn), cotton, and wheat caused by these stresses represent a significant economic and political factor and contribute to food shortages in many underdeveloped countries.
Water availability is an important aspect of the abiotic stresses and their effects on plant growth. Continuous exposure to drought conditions causes major alterations in the plant metabolism which ultimately lead to cell death and consequently to yield losses. Because high salt content in some soils results in less water being available for cell intake, high salt concentration has an effect on plants similar to the effect of drought on plants. Additionally, under freezing temperatures, plant cells lose water as a result of ice formation within the plant. Accordingly, crop damage from drought, heat, salinity, and cold stress, is predominantly due to dehydration.
Because plants are typically exposed to conditions of reduced water availability during their life cycle, most plants have evolved protective mechanisms against desiccation caused by abiotic stresses. However, if the severity and duration of desiccation conditions are too great, the effects on development, growth, plant size, and yield of most crop plants are profound. Developing plants efficient in water use is therefore a strategy that has the potential to significantly improve human life on a worldwide scale.
Traditional plant breeding strategies are relatively slow and require abiotic stress-tolerant founder lines for crossing with other germplasm to develop new abiotic stress-resistant lines. Limited germplasm resources for such founder lines and incompatibility in crosses between distantly related plant species represent significant problems encountered in conventional breeding. Breeding for tolerance has been largely unsuccessful.
Many agricultural biotechnology companies have attempted to identify genes that could confer tolerance to abiotic stress responses, in an effort to develop transgenic abiotic stress-tolerant crop plants. Although some genes that are involved in stress responses or water use efficiency in plants have been characterized, the characterization and cloning of plant genes that confer stress tolerance and/or water use efficiency remains largely incomplete and fragmented. To date, success at developing transgenic abiotic stress-tolerant crop plants has been limited, and no such plants have been commercialized.
In order to develop transgenic abiotic stress-tolerant crop plants, it is necessary to assay a number of parameters in model plant systems, greenhouse studies of crop plants, and in field trials. For example, water use efficiency (WUE), is a parameter often correlated with drought tolerance. Studies of a plant's response to desiccation, osmotic shock, and temperature extremes are also employed to determine the plant's tolerance or resistance to abiotic stresses. When testing for the impact of the presence of a transgene on a plant's stress tolerance, the ability to standardize soil properties, temperature, water and nutrient availability and light intensity is an intrinsic advantage of greenhouse or plant growth chamber environments compared to the field.
WUE has been defined and measured in multiple ways. One approach is to calculate the ratio of whole plant dry weight, to the weight of water consumed by the plant throughout its life. Another variation is to use a shorter time interval when biomass accumulation and water use are measured. Yet another approach is to use measurements from restricted parts of the plant, for example, measuring only aerial growth and water use. WUE also has been defined as the ratio of CO2 uptake to water vapor loss from a leaf or portion of a leaf, often measured over a very short time period (e.g. seconds/minutes). The ratio of 13C/12C fixed in plant tissue, and measured with an isotope ratio mass-spectrometer, also has been used to estimate WUE in plants using C3 photosynthesis.
An increase in WUE is informative about the relatively improved efficiency of growth and water consumption, but this information taken alone does not indicate whether one of these two processes has changed or both have changed. In selecting traits for improving crops, an increase in WUE due to a decrease in water use, without a change in growth would have particular merit in an irrigated agricultural system where the water input costs were high. An increase in WUE driven mainly by an increase in growth without a corresponding jump in water use would have applicability to all agricultural systems. In many agricultural systems where water supply is not limiting, an increase in growth, even if it came at the expense of an increase in water use (i.e. no change in WUE), could also increase yield. Therefore, new methods to increase both WUE and biomass accumulation are required to improve agricultural productivity.
Concomitant with measurements of parameters that correlate with abiotic stress tolerance are measurements of parameters that indicate the potential impact of a transgene on crop yield. For forage crops like alfalfa, silage corn, and hay, the plant biomass correlates with the total yield. For grain crops, however, other parameters have been used to estimate yield, such as plant size, as measured by total plant dry weight, above-ground dry weight, above-ground fresh weight, leaf area, stem volume, plant height, rosette diameter, leaf length, root length, root mass, tiller number, and leaf number. Plant size at an early developmental stage will typically correlate with plant size later in development. A larger plant with a greater leaf area can typically absorb more light and carbon dioxide than a smaller plant and therefore will likely gain a greater weight during the same period. This is in addition to the potential continuation of the micro-environmental or genetic advantage that the plant had to achieve the larger size initially. There is a strong genetic component to plant size and growth rate, and so for a range of diverse genotypes plant size under one environmental condition is likely to correlate with size under another. In this way a standard environment is used to approximate the diverse and dynamic environments encountered at different locations and times by crops in the field.
Harvest index, the ratio of seed yield to above-ground dry weight, is relatively stable under many environmental conditions and so a robust correlation between plant size and grain yield is possible. Plant size and grain yield are intrinsically linked, because the majority of grain biomass is dependent on current or stored photosynthetic productivity by the leaves and stem of the plant. Therefore, selecting for plant size, even at early stages of development, has been used as to screen for plants that may demonstrate increased yield when exposed to field testing. As with abiotic stress tolerance, measurements of plant size in early development, under standardized conditions in a growth chamber or greenhouse, are standard practices to measure potential yield advantages conferred by the presence of a transgene.
There is a need, therefore, to identify additional genes expressed in stress tolerant plants and/or plants that are efficient in water use that have the capacity to confer stress tolerance and/or increased water use efficiency to the host plant and to other plant species. Newly generated stress tolerant plants and/or plants with increased water use efficiency will have many advantages, such as an increased range in which the crop plants can be cultivated, by for example, decreasing the water requirements of a plant species. Other desirable advantages include increased resistance to lodging, the bending of shoots or stems in response to wind, rain, pests, or disease.